This invention relates to improvements in transistorized static inverters for converting direct voltage into alternating voltage and supplying energy to an output load. The invention is particularly adapted for operating fluorescent lamps, although loads of other types, such as integrated circuits, are also acceptable.
Transistorized inverters, frequently designated "electronic ballast" are broadly used for operating fluorescent lamps. Examples of such inverters are disclosed in U.S. Pat. No. 3,219,946 (Compoly), Nos. 3,448,370 (Harrigan), and 3,500,168 (Merritt), as well as in French patent specifications 2,165,037 and 1,359,664. Most of the existing inverters have substantial drawbacks. They are not suitable for operating loads which require an output power which may vary within broad limits while correctly operating within the complete range. However, such conditions are frequently encountered. For instance, when an inverter is used for operating a plurality of fluorescent lamps, some of the lamps may be cut off or failed. Also, most static inverters are so constructed that their transistors are destructed if a short-circuit should develop across the output of the inverter. For overcoming that difficulty, U.S. Pat. No. 3,219,946 suggests a circuit having a voltage divider network which tends to reduce the bias voltage of the transistors upon occurence of a short-circuited output condition. That network and the associated coils may be considered as analogous to a pilot circuit. It does not however permit satisfactory operation under highly variable output conditions and particularly highly variable output current values.
It is an object of the invention to provide a transistorized static inverter which permits operation under highly variable output conditions.
It is an other object of the invention to provide a static inverter for simultaneously operating a plurality of loads, and particularly of fluorescent lamps, while allowing for simultaneous adjustment of the RMS voltage and frequency applied to all loads.
For that purpose, there is provided a static inverter comprising:
A. a first and a second switching transistors each having emitter, collector and base, PA1 B. a first resistor and a second resistor respectively connecting the bases of said first and second transistors to a junction point and operating as current generator for said transistors, PA1 C. a source of DC voltage connected to said junction point, PA1 D. inductance means inductively coupling said bases, PA1 E. first and second coils inductively coupled to said inductance means and respectively connecting the collectors of said transistors to said junction, PA1 F. capacitor means connecting said collectors, said capacitor means, coils and inductance means constituting a frequency pilot circuit for said transistors, PA1 g. first and second switching rectifier means connecting the collectors of said transistors to ground for uncoupling said collectors, PA1 h. and at least one output transformer, each said transformer having a primary connected across said collectors and a secondary whose ends constitute terminals for connection of an output load.
Self-inductance coil means may preferably be connected in series between the junction and the DC source. That self-inductance should then have an impedance high enough for impedance matching of the source and the transistor circuit, that is a value which is much higher than that of the filter coils which have sometimes been used.
The invention may be used for operating fluorescent lamps at a frequency which may be as high as 200 kHz and is adjustable by modifying the number of turns of the coils of the pilot circuit. PNP transistors may be substituted for NPN transistors, subject to polarity changes.
In a particular embodiment of the invention, each primary consists of two identical windings connected in series relation, electric conductor means connecting the common ends of said windings to said junction and respective rectifier means connecting said collectors to the remaining ends of said windings.